OK, here are some pictures.
First, the front of the cabinet, showing the minimal support for the granite lip:
Then from the top, showing where the L/H cut line will be.
Then a view under the front lip, showing how much ( or little! ) wood there is:
Then a view of the top front. The forward edge of the level is right on the cut line.
Here's the strip of aircraft grade plywood to fit in the slot in front:
Strip of plywood sliding into the lip:
Strip of plywood farther into the lip:
Strip of plywood loosely captured by false front. It is resting on top of the false front, which becomes a structural member. The false front will transfer load from the middle of the granite lip to the bottom edges of that particular cabinet bay. By "edges", I mean the L/H and R/H edges. That's where the cabinet is stiff. There will be a piece of 1/2" or 3/4" plywood completely filling the drawer hole that the false front covers.
False front rammed home up against the plywood strip. Note that the gap between the false front and the actual drawer under it is proper. The plywood strip disappears almost completely.
I will also epoxy the plywood strip into the slot. Yeah, it's belt and suspenders.
The existing front lip is 3 7/8" wide. I will have to cut it down to 2 5/8".
The bullnose is 1 9/16" thick, so the granite itself is half of that, or approx 20mm.
The existing wood under that lip is 2.05" wide. After the cut, there will be 0.925" left. With my quarter inch of plywood, that gets us 1.175 inches of support. Call it 1 1/8".